1. Field of the Invention
The present invention relates to a continuous casting nozzle for permitting effective prevention of narrowing of, clogging of or metal sticking to the nozzle bore through which molten steel passes when performing continuous casting of molten steel containing aluminum such as aluminum-killed steel.
2. Description of the Related Art
A continuous casting nozzle for casting molten steel is used for the following purposes. As for continuous casting molten steel, a continuous casting nozzle is used for the purpose of preventing the molten steel from being oxidized by contacting the open air, from splashing when the molten steel is poured from a tundish to a mold, and rectifying the flow of the molten steel poured for preventing non-metallic inclusion and slag present near or on the mold surface from being entrapped in the cast steel strand.
Material of a conventional continuous casting nozzle of molten steel comprises such material as graphite, alumina, silica, silicon carbide and recently zirconia. However, there are the following problems in the case of casting aluminum-killed steel and the like.
As for the aluminum-killed steel and the like, aluminum, which is added as a deoxidizer, it reacts with oxygen existing in the molten steel to produce non-metallic inclusion such as alumina. Therefore, in casting the aluminum-killed steel and the like, the non-metallic inclusion such as the alumina adheres and accumulates onto the surface of the bore of the continuous casting nozzle, so that the bore is narrowed or clogged up in the worst case, which makes stable casting difficult. Furthermore, the non-metallic inclusion such as the alumina adhered or accumulated onto the surface of the bore peels off or falls down, and is entrapped in the cast steel strand, thus degrading the quality of the cast steel strand.
For the purpose of preventing the above-mentioned reduction or clogging of the bore caused by the non-metallic inclusion such as alumina, there is a commonly used method for preventing the non-metallic inclusion such as alumina existing in the molten steel from adhering or accumulating onto the surface of the bore of the nozzle by ejecting inert gas from the inner surface of the nozzle bore toward the molten steel flowing through the bore. One example of this method is described in Japanese Patent Publication No. Hei 6-59533/1994.
However, the method wherein the inert gas is ejected from the inner surface of the nozzle forming the bore has the following problem. A large amount of the ejected inert gas causes entrapment of bubbles produced by the inert gas into the cast steel strand, resulting in defects based on needle-like gas bubbles or pinholes. On the other hand, a small amount of the ejected inert gas causes adhesion and accumulation of the non-metallic inclusion such as the alumina onto the surface of the bore of the nozzle, thus, in the worst case, causing narrowing or clogging of the bore.
Additionally, it is constructionally difficult to uniformly eject the inert gas from the inner surface of the nozzle bore toward the molten steel flowing through the bore. In the instance where the casting is performed for a long period of time, a stable control of the amount of ejected inert gas becomes gradually more difficult, as the structure and the structure of the material consisting of the continuous casting nozzle degrades. And moreover, it becomes difficult to eject inert gas uniformly from the inner surface to the nozzle bore. As a result, the non-metallic inclusion such as the alumina adheres and accumulates onto the surface of the bore of the nozzle so that the bore is narrowed or clogged at the end.
It is thought that the clogging of the nozzle by the non-metallic inclusion, especially by the alumina inclusion, is caused as described below.
(1) Alumina inclusion is produced from aluminum existing in the steel by secondary oxidation, such as oxidation by entrapped air passing through a refractory junction and refractory structure or oxidation by supplying oxygen obtained from reduction of silica in a graphite or carbon-containing refractory.
(2) Alumina inclusion is produced by diffusion and cohesion of the alumina produced in the above process.
(3) Graphite on the surface of the nozzle bore vanishes and the surface of the bore becomes rough and the alumina inclusion is apt to accumulate on the rough surface of the bore.
On the other hand, as an alternative in view of the nozzle material, a nozzle having a non-oxide raw material (SiC, Si.sub.3 N.sub.4, BN, ZrB.sub.2, SIALON, etc.) with low reactivity with aluminum oxide is added to alumina-graphite or a nozzle consisting of the non-oxide material itself is proposed. An example of this alternative is described in Japanese Patent Publication No. Sho 61-38158/1986.
However, this alternative is not practical in the case of the alumina-graphite nozzle, because the adhesion preventing effect is not recognized and further corrosion resistance is decreased unless a large amount of the non-oxide material is added.
Also, a nozzle only consisting of the non-oxide material is not suitable for practical use in view of material cost and manufacturing cost, although a substantial effect is expected.
A nozzle consisting of graphite-oxide raw material containing CaO is proposed for producing low-melting-point material by a reaction of CaO in an oxide raw material containing CaO (CaO.multidot.ZrO.sub.2, CaO.multidot.SiO.sub.2, 2CaO.multidot.SiO.sub.2, etc.) with Al.sub.2 O.sub.3 and forming the low-melting-point material in the steel. An example of this type of nozzle is described in Japanese Patent Laid-Open Publication No. Sho 62-56101.
However, reactivity of CaO with Al.sub.2 O.sub.3 is apt to be influenced by the temperature of the molten steel when casting, and in one instance, the amount of CaO is not sufficiently secured for satisfying spalling resistance and corrosion resistance when a sufficient amount of Al.sub.2 O.sub.3 inclusion is contained in the steel.